Metal sheets such as tinned steel sheets and electrolytically chromate-treated steel sheets, which are called tin-free steel sheets, have been used for various metal cans such as beverage cans, food cans, pails, and 18 L cans. Among others, tin-free steel sheets, which are manufactured by electrolytic treatment of steel sheets in a plating bath containing hexavalent Cr, have the advantage of excellent humid resin adhesion to a resin such as a coating.
Recently, the growing environmental awareness has resulted in regulations on the use of hexavalent Cr worldwide, and there is a need for an alternative to tin-free steel sheets, which are manufactured using a plating bath containing hexavalent Cr.
In conventional metal can manufacturing, metal sheets such as tin-free steel sheets are coated and formed into cans. Recently, however, resin-covered metal sheets, which are covered with a resin such as a plastic film, rather than coated, have increasingly been used in can forming for reduced waste during manufacture. Such resin-covered metal sheets require the resin to adhere strongly to the metal sheets. In particular, resin-covered metal sheets used for beverage cans and food cans, which may undergo a retort sterilization process after being filled with the contents thereof, require sufficient humid resin adhesion for the resin not to peel off in a hot, humid environment and sufficient corrosion resistance not to be corroded and perforated by, for example, the contents of the cans after the resin comes off partially due to, for example, scratching.
To meet such a need, the inventors have recently disclosed in patent document 1 that a Cr-free surface-treated steel sheet with significantly excellent humid resin adhesion and excellent corrosion resistance can be manufactured by forming a corrosion-resistant coating composed of at least one layer selected from a Ni layer, a Sn layer, an Fe—Ni alloy layer, an Fe—Sn alloy layer, and an Fe—Ni—Sn alloy layer on at least one side of a steel sheet and then forming an adherent coating by cathode electrolytic treatment in an aqueous solution containing an ion having Ti and an ion having at least one metallic element selected from Co, Fe, Ni, V, Cu, Mn, and Zn.